The present invention relates to an improved method for producing microcapsules and, more particularly, to an improved method for producing microcapsules in which the wall is formed from a formaldehyde condensation product. While the method of the present invention is useful in producing microcapsules generally, microcapsules prepared in accordance with the present invention are particularly useful in photosensitive imaging materials of the type which employ a layer of microcapsules containing a radiation sensitive composition.
Imaging materials employing photosensitive microcapsules are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,836. Imaging materials employing other types of photoactive microcapsules are the subject of the following U.S. patents.
Berman, U.S. Pat. No. 3,219,446 discloses a transfer imaging process in which a blue-black dye is encapsulated with a photocrosslinkable polymer or a photopolymerizable monomer in a film or a layer of discrete microcapsules. In this system imaging is accomplished by image-wise exposing the layer of the encapsulated material to electromagnetic radiation to crosslink the polymer or polymerize the monomer. This is said to cause the liquid in the exposed capsules to assume a non-liquid, rigid condition, such that upon rupturing the capsules only the dye from the unexposed capsules is transferred to a receiving sheet where images are formed.
Phillips, U.S. Pat. No. 3,700,439, discloses a process wherein Michler's ketone is encapsulated in a conventional manner and provided as a layer on a support. Michler's ketone is not a color former, but irradiation of the ketone converts it to a colorless, acid colorable, dye precursor. Thus, by image-wise exposing the Phillips material to actinic radiation and rupturing the capsules in contact with an acid developer layer, such as a layer of acid clay, a visible image is obtained in the areas in which the ketone has been converted to the acid colorable form by irradiation. Phillips also discloses a self-contained system in which the acid developer is on the same support as the capsules containing the Michler's ketone.
Levy, U.S. Pat. No. 4,149,887, relates to a capsule-containing imaging material having a photoconductive internal phase which is exposed in an RF field. Exposure increases the conductivity of the internal phase and causes the capsules to heat, swell and then rupture in the exposed areas thereby freeing the contents to form images. The capsules may contain color precursors which form images by reacting with a developing agent.
Japanese Kokai, 6212/1974 to Matsushita Denki Sangyo K.K., relates to a recording material in which microcapsules having a photocrosslinkable polymer wall contain a colorless or substantially colorless precursor in a polar solvent and, optionally, an acid dye developer. The polar solvent prevents the color precursor from reacting with the acid dye developer and forming a color image. Images are formed by image-wise exposing the material to actinic radiation and rupturing the capsules in the unexposed areas, whereupon the polar solvent evaporates and color development occurs.
Japanese Kokai, 9519 and 9520/1978 to K.K. Rikoh, relate to imaging systems in which the capsules contain a radiation curable internal phase including a dye precursor. Images are formed by image-wise exposing a layer of the capsules to light and selectively transferring the capsules from the unexposed areas to a receiving sheet. The capsules are not ruptured. Upon subsequent wet development processing of the transfer sheet with a dye developer, the dye precursor in the capsules is converted to a colored form and an image is obtained.
British Pat. Nos. 1,001,832; 1,058,798; 1,141,475; and 1,193,923 to DuPont relate to imaging materials which employ a photopolymerizable substratum comprising a solid binder having a polymerizable ethylenically unsaturated compound dispersed therein. Images are formed by image-wise exposing the materials to actinic radiation. Exposure controls the rate of diffusion of a developer into or out of the photopolymerizable substratum. In the case of British Pat. No. 1,058,798, a dye or color precursor is present in the capsules and images are formed by bringing a receiving sheet, such as plain or coated paper, into contact with a substratum and heating the two in combination such that the dye diffuses from the unexposed areas of the substratum to the receiving sheet. In the case of British Pat. No. 1,141,475, at least one of a color former, developer and oxidizing agent is present in the dispersed phase and the substratum is hardened in the exposed areas thereby altering the rate of diffusion of an externally applied reactant into the substratum.
In the aforesaid commonly assigned U.S. patents, images are formed by image-wise exposing a layer of photosensitive capsules to actinic radiation and rupturing the capsules typically by passing the imaging sheet containing the capsules through a pressure nip. The radiation sensitive composition contains a photohardenable or photosoftenable material which undergoes a change in viscosity upon exposure. For example, in the most typical embodiments, the radiation sensitive composition contains a polyethylenically unsaturated monomer which polymerizes upon exposure, thereby causing the phase internal to the capsules to become harder. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules rupture and release their contents. If the internal phase contains a dye precursor, the precursor is image-wise released, and a color image is formed upon its transfer to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules or a separate support. It is advantageous if the developer is present on the same support since such a self-contained imaging sheet can be developed as an integral unit.
One of the problems associated with self-contained imaging sheets is that smudging and pinpoints occur as the sheet is handled, packaged or stacked for storage. This is due to the capsules inadvertently rupturing and releasing the dye precursor which reacts with the developer present on the sheet. The problem intensifies if the capsules are too large and can be eliminated or substantially lessened if the capsules are reduced in size. However, previously it has only been possible to obtain capsules having the desired uniform small size using a certain dye precursor. This is not an acceptable solution to the problem, because it is often necessary to use other dye precursors in the capsules to form images of other colors or to improve the color tone.
Thus, there is a need for an improved method for producing microcapsules of uniform small particle size which are particularly useful in providing a self-contained imaging sheet.